The present invention relates generally to radio frequency (RF) power amplifiers, and more specifically to a circuit for causing a bias current in the RF power amplifier to be set and maintained at a predetermined fixed value.
Radio frequency (RF) power amplifiers characterized by a plurality of operating performance characteristics responsive to a quiescent operating point established by a direct current (DC) bias current are used in a wide variety of communications and other electronic applications. These amplifiers are made up of one or more cascaded amplifier stages, each of which increases the level of the signal applied to the input of that stage by an amount known as the stage gain. Ideally, the input to output transfer of each stage is linear, i.e., a perfect replica of the input signal increased in amplitude appears at the amplifier output. In reality, however, all power amplifiers have a degree of non-linearity in their transfer characteristic. This non-linearity adversely affects various amplifier operating characteristics such as gain performance, intermodulation performance and efficiency.
The optimal quiescent operating point of the RF power amplifier and, thereby, the optimal DC bias current is a critical design for optimal linearity in the RF power amplifier. Once the optimal bias current in the RF power amplifier is set, it is desirable to maintain the optimal bias current in the RF power amplifier. However, the bias current typically drifts from its optimal point over time as a function of factors such as temperature variation, process variation, and history of the RF power amplifier.
One technique for maintaining the optimal DC biasing point is referred to as a self-bias technique, in which a portion of the output signal of the RF power amplifier is used as a feedback to adjust the bias point of the amplifier. This self-bias technique adversely affects the performance of the RF power amplifier and is inappropriate for high performance RF power amplifiers.
Another widely practiced technique to maintain the optimal DC biasing point is an active bias tuning technique. This technique can adjust DC biasing points according to process variations of a device, but it cannot adjust the DC biasing points in response to temperature variations and history of the device. In addition, such tuning is expensive and time consuming.
Yet another technique is resetting the biasing points after burning in the device. This technique can often lessen but not eliminate the problem of a DC biasing point drifting as the device ages, but the burning process is time consuming and costly.
A large DC resistance connected in series with the emitter of a bipolar transistor is one technique that may be used to reduce the temperature sensitivity of the transistor. However, the voltage drop across and the power loss in the large resistance adversely affect the RF power amplifier containing the transistor.
Still another technique uses a microprocessor controlled active bias control circuit to periodically reset the DC bias points. However, this technique is complicated and expensive.
Yet another technique is disclosed in U.S. Pat. No. 6,046,642, entitled AMPLIFIER WITH ACTIVE BIAS COMPENSATION AND METHOD FOR ADJUSTING QUIESCENT CURRENT. The technique and circuit disclosed for maintaining the optimal bias current in the RF power amplifier addresses many of the shortcomings of the above prior art. However, this technique requires that the active bias compensation circuit be off-chip from the RF power amplifier, and like the above prior art, this technique does not allow for suppression of RF and baseband energy that may build up on a reference transistor. In addition, the circuit disclosed in U.S. Pat. No. 6,046,642 is not as space efficient, nor as cost effective or as efficient in power consumption as one would desire.
Thus, there exists a need for a simple, space effective, power effective, and cost efficient circuit for adjusting a bias current so that an optimal quiescent operating point is maintained in an RF power amplifier over factors such as temperature variation, process variation, and history of the RF power amplifier, and that performs thermal tracking and suppression of RF and baseband energy as needed, and that will not require any tuning in a manufacturing environment.